Every living organism is constantly challenged by infectious or pathogenous agents such as bacteria, viruses, fungi or parasites. The so-called immune system prevents the organism from permanent infections, diseases or intoxication caused by such agents.
The immune system of a mammal can be divided into a specific and an unspecific part although both parts are closely cross-linked. The unspecific immune response enables an immediate defense against a wide variety of pathogenic substances or infectious agents. The specific immune response is raised after a lag phase, when the organism is challenged with a substance for the first time. This specific immune response is mainly based on the production of antigen-specific antibodies and the generation of macrophages and lymphocytes, e.g. cytotoxic T-cells (CTL). The specific immune response is responsible for the fact that an individual who recovers from a specific infection is protected against this specific infection but still is susceptible for other infectious diseases. In general, a second infection with the same or a very similar infectious agent causes much milder symptoms or no symptoms at all. This so-called immunity persists for a long time, in some cases even lifelong. The underlying effect is often referred to as immunological memory, which can be used for vaccination proposes.
With the term vaccination a method is described, where an individual is challenged with a harmless, partial or inactivated form of the infectious agent to affect, preferably induce, an immunological response in said individual, which leads to long lasting—if not lifelong—immunity against the specific infectious agent.
The human smallpox disease is caused by Variola virus: Variola virus belongs to the family of Poxviridae, a large family of complex DNA viruses that replicate in the cytoplasma of vertebrate and invertebrate cells.
The family of Poxviridae can be divided into the two subfamilies Chordopoxvirinae and Entomopoxvirinae based on vertebrate and insect host range. The Chordopoxvirinae comprise beside others the genera of Orthopoxviruses and Avipoxviruses (Fields Virology, ed. by Fields B. N., Lippincott-Raven Publishers, 3rd edition 1996, ISBN: 0-7817-0253-4, Chapter 83).
The genera of Orthopoxviruses comprises variola virus, the causative agent of human smallpox, and also other viruses with economical importance, e.g. camelpox, cowpox, sheeppox, goatpox, monkeypox and Vaccinia virus. All members of this genus are genetically related and have similar morphology or host range. Restriction endonuclease maps have even shown high sequence identity from up to 90% between different members of the Orthopoxviruses (Mackett & Archard, [1979], J Gen Virol, 45: 683-701).
Vaccinia virus (VV) is the name given to the agent that was used at least the last 100 years for the vaccination against smallpox. It is not known whether VV is a new species derived from cowpox or variola virus by prolonged serial passages, the living representative of a now extinct virus or maybe a product of genetic recombination. Additionally, in course of the VV history, many strains of Vaccinia have arisen. These different strains demonstrate varying immunogenicity and are implicated to varying degrees with potential complications, the most serious of which is post-vaccinial encephalitis. However, many of these strains were used for the vaccination against smallpox. For example the strains NYCBOH, Western Reserve or Wyeth were used primarily in US, while the strain Ankara, Bern, Copenhagen, Lister and MVA were used for vaccination in Europe. As a result of the worldwide vaccination program with these different strains of VV in 1980 the WHO finally declared the successful eradication of variola virus.
Today, VV is mainly used as a laboratory strain, but beside this it is still considered as the prototype of Orthopoxviruses, which is also the reason why VV became one of the most intensively characterized viruses (Fields Virology, ed. by Fields B. N., Lippincott-Raven Publishers, 3rd edition 1996, ISBN: 0-7817-0253-4, Chapter 83 and 84).
VV and more recently other poxviruses have been used for the insertion and expression of foreign genes. The basic technique for inserting foreign genes into live infectious poxvirus involves recombination between pox DNA sequences flanking a foreign genetic element in a donor plasmid and homologous sequences present in the rescuing poxvirus. Genetic recombination is, in general, the exchange of homologous sections of DNA between two strands of DNA. In certain viruses RNA may replace DNA. Homologous sections of nucleic acid are sections of nucleic acid (DNA or RNA), which have the same sequence of nucleotide bases. Genetic recombination may take place naturally during the replication or manufacture of new viral genomes within an infected host cell. Thus, genetic recombination between viral genes may occur during the viral replication cycle that takes place in a host cell, which is co-infected with two or more different viruses or other genetic constructs. A section of DNA from a first genome is used interchangeably in constructing the section of the genome of a second co-infecting virus in which the DNA is homologous with that of the first viral genome.
Successful expression of the inserted DNA genetic sequence by the modified infectious virus requires two conditions. First, the insertion should be into a nonessential region of the virus in order that the modified virus remains viable. The second condition for expression of inserted DNA is the presence of a promoter in the proper relationship to the inserted DNA. Regularly, the promoter is located upstream from the DNA sequence to be expressed.
The usefulness of recombinant VV expressing, e.g., Hepatitis B virus surface antigen (HBsAg), Influenza virus hemagglutinin (InfHA) or Plasmodium knowlesi sporozoite antigen, as live vaccines for the prophylaxis of infectious diseases has been demonstrated and reviewed (Smith, et al. [1984] Biotechnology and Genetic Engineering Reviews 2, 383-407).
A further advantage of VV is the capacity to take up multiple foreign sequences, genes or antigens within a single VV genome (Smith & Moss [1983], Gene, 25(1): 21-28). Furthermore, it has been reported that it is possible to elicit immunity to a number of heterologous infectious diseases with a single inoculation of a polyvalent vaccine (Perkus et al., [1985], Science, Vol. 229, 981-984).
One example of the expression of various antigens by a single VV is described by Bray et al. It was shown that a recombinant VV, which is capable to express three different structural proteins of Dengue virus serotype 4, namely the capsid (C), pre-membrane (prM), envelope (E) protein, and two non-structural proteins of Dengue virus serotype 4, namely NS1 and NS2a, had the ability to protect mice against a homologous Dengue virus serotype 4 challenge (Bray et al., [1989], Virology 2853-2856).
The Dengue virus with its four serotypes, Dengue virus serotype 1 (Den-1) through Dengue virus serotype 4 (Den-4), is one important member of the Flavivirus genus with respect to infections of humans. Dengue virus infection produces diseases that range from flu-like symptoms to severe or fatal illness, Dengue haemorrhagic fever (DHF) with shock syndrome (DSS). Dengue outbreaks continue to be a major public health problem in densely populated areas of the tropical and subtropical regions, where mosquito vectors are abundant.
The concern over the spread of Dengue infection and other diseases induced by mosquito-borne Flaviviruses in many parts of the world has resulted in more efforts being made towards the development of Dengue vaccines, which could prevent both Dengue fever (DF), and Dengue haemorrhagic fever (DHF) and in vaccines useful to protect the vaccinated individual against infections induced by some or all mosquito-borne flaviviruses. 
While most cases of DF are manifested after the first infection by any of the four serotypes, a large percentage of DHF cases occur in subjects who are infected for the second time by a serotype, which is different from the first infecting serotype of Dengue virus. These observations give rise to the hypothesis that sequential infection of an individual having antibodies against one Dengue serotype by a different virus serotype at an appropriate interval may result in DHF in a certain number of cases.
Accordingly, vaccination against one serotype does not result in a complete protection against Dengue virus infection, but only against infection with the same Dengue virus strain. Even more important, a person vaccinated against one serotype, has an increased risk of developing severe complications such as Dengue haemorrhagic fever when said person is infected from a Dengue virus strain of a different serotype.
Thus, a multivalent vaccine that contains antigens from all four Dengue virus serotypes is desired.
So far it had been suggested to prepare multivalent vaccines by mixing a panel of recombinant VV, each VV encoding sequences of a different viruses (Moss, [1990] Immunology, 2, 317-327). However, such a multivalent vaccine comprises several disadvantages. Firstly, it is cumbersome to generate several independent recombinant VV. Beside the separated production processes, also quality control and quality assurance is highly time consuming. Secondly, an infection with a mixture of recombinant viruses expressing different sequences always bears the risk that the infection event is not particularly well balanced. The main risk is that only individual recombinants, but not all different recombinants comprised in the multivalent vaccine will infect target cells. One reason might be an uneven distribution of recombinant viruses. Another reason might be interferences between the different recombinant viruses while infecting single cells. Such interferences are known as phenomenon of superinfection. In this case, only some antigens, but not all different antigens of the multivalent vaccine will finally be expressed from infected cells and, thus, presented to the immune system of a patient. As a consequence, immune protection will be obtained only against some of the antigens, but is far from providing a complete immune protection against the various antigens presented or presentable by the multivalent vaccine.
In the context of a vaccine against Dengue virus infection the approach of a multivalent vaccine has the disadvantage that if the different sequences are expressed in different amounts or in an unpredictable manner, as it had been shown for the envelope protein of Dengue virus 2 (Deuble et al., [1988], J. Virol 65: 2853), then such a vaccination is highly risky for a patient. An incomplete vaccination using a panel of recombinant Vaccinia viruses will only provide an immune protection against some, but not against all serotypes of Dengue virus. Unfortunately, in case of Dengue infection an incomplete vaccination is extremely unacceptable, since it increases the risk of lethal complications such as Dengue hemorrhagic fever.